[Rcpp-devel] Rcpp equivalent of sapply(L, length)

[Qiang Kou]

What about using "Rf_length"?

-------------------------------------------------------
#include <Rcpp.h>

using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector c_listlengths(List L) {
    IntegerVector lens(L.size());

    for (int i=0; i<L.size(); i++) {
        SEXP x = L[i];
        lens(i) = Rf_length(x);
    }
    return lens;
}
----------------------------------------------------------

In R:

> Rcpp::sourceCpp("example.cpp")
> l=list(1:3, 2:3, 1:6)
> l2=list(letters[1:3], letters[2:3], letters[1:6])
> c_listlengths(l)
[1] 3 2 6
> c_listlengths(l2)
[1] 3 2 6
> l3=list(letters[1:6], letters[2:3], letters[1:3])
> c_listlengths(l3)
[1] 6 2 3


On Thu, Jul 1, 2021 at 5:14 PM Dr Gregory Jefferis <
jefferis at mrc-lmb.cam.ac.uk> wrote:

> Dear RcppUsers,
>
> I feel like there must be a simple way to do the equivalent of
>
> sapply(L, length)
>
> in Rcpp but I can't seem to get past the problem of converting the
> elements of L into a type for which .size() or .length() are valid. I
> see that I could do this using some long switch(TYPEOF(x)) statement (eg
> https://gallery.rcpp.org/articles/rcpp-return-macros/):
>
> // [[Rcpp::export]]
> IntegerVector c_listlengths(List L) {
>    IntegerVector lens(L.size());
>
>    for (int i=0; i<L.size(); i++) {
>      SEXP x = L[i];
>      switch (TYPEOF(x)) {
>      case INTSXP: {
>        lens(i)=as<IntegerVector>(x).size();
>      }
>         // handle other SXP types
>      }
>    }
>    return lens;
> }
>
> but that that seems horribly convoluted. Can anyone point me to a
> simpler way?
>
> Apologies if this is an FAQ, but I did not yet manage to turn up
> anything close enough.
>
> With many thanks,
>
> Greg.
>
> # R example
> l=list(1:3, 2:3, 1:6)
> l2=list(letters[1:3], letters[2:3], letters[1:6])
> sapply(l, length)
> sapply(l2, length)
>
>
>
> --
> Gregory Jefferis
> Division of Neurobiology
> MRC Laboratory of Molecular Biology
> Francis Crick Avenue
> Cambridge Biomedical Campus
> Cambridge, CB2 OQH, UK
>
> http://www2.mrc-lmb.cam.ac.uk/group-leaders/h-to-m/g-jefferis
> http://jefferislab.org
> https://www.zoo.cam.ac.uk/research/groups/connectomics
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